1. Field of the Invention
This invention relates to phosphate ester base stock compositions having a novel combination of phosphate ester components and to aircraft hydraulic fluid compositions comprising such base stocks.
2. State of the Art
Hydraulic fluids used in the hydraulic systems of aircraft must meet exacting specifications set by aircraft manufacturers. Accordingly, the components of aircraft hydraulic fluids are carefully chosen to balance, among other properties, stability, compatibility, density, toxicity and the like. Whether the selected components can, in fact, be balanced to meet these specifications is unpredictable. Moreover, the amounts of individual components used in compositions which meet the specifications is not a priori predictable.
It has now been discovered that a particular combination of phosphate ester components employed in the base stock of aircraft hydraulic fluid compositions provides surprising and unexpected properties. Specifically, it has been found that by selecting particular ratios of the tri-iso-butyl and tri-n-butyl phosphate ester components of the fluid, an unexpected and surprising balance of combined properties critical to aviation hydraulic oils is obtained, including acceptable hydrolytic stability, high flash point, good anti-wear properties, acceptable erosion protection, acceptable low temperature flow properties (viscosity), and elastomer compatibility.